# How close could a gas giant be to my planet without it becoming a moon?

This is an M-type system. Let's take Trappist 1 as an example star (0.0898 Mass of the Sun) and Trappist 1e as an example planet (0.772 mass of the earth, approx 0.029 AU from the star). Assume for now that none of the other Trappist rocky planets exist.

Now put a gas giant equivalent in size and mass to Jupiter as the outer planet of the system. I know gas giants are extremely unlikely in m-type systems, but stranger things have happened in the universe.

How close could the gas giant orbit the star without destabilizing 1e's orbit or turning it into a moon of the gas giant?

A Jupiter mass planet orbiting a star with 0.0898 solar masses would be problematic.

Jupiter has a mass of 0.00095 solar masses, which is just 100 times less than the central star you have chosen. What does it mean?

For the solar system, where the Sun has 99.8% of the total mass of the system, the center of mass of the system lays just outside the Sun's surface, depending on Jupiter's position.

For your system, the center of mass would be way out of the star, meaning that the star itself would visibly wobble around it.

In numbers, the center of mass of the system would be at about 1% of the distance between the star and the Jupiter-like planet, starting from the star.

This means that sooner or later your planet, laying at just 0.029 AU from the star, would either sunk into it or be kicked out of the system due to the gravitational influences of the two bodies.

• Gotcha. Thanks! Could a smaller gas planet be plausible? Commented Mar 17, 2023 at 15:18
• Yes. In fact, there are smaller gas giants in our own system (in fact, all of our gas giants are smaller than Jupiter except for Jupiter). Commented Mar 17, 2023 at 16:07
• @L.Dutch To follow up, what would be the threshold ratio of star mass to planet mass for a system with stable orbital mechanics? Commented Mar 21, 2023 at 17:08